This invention is related to the synthesis and preparation of novel materials for use as strong permanent hard magnets. Many of today's advancing technologies require an efficient and strong hard magnet as a basic component of the device structure. Such devices range from cellular phones to high performance electric motors and significant effort is ongoing throughout the industry to find materials which not only meet current requirements, but also ever increasing demand for efficient, less expensive and easily produced hard magnet materials.
Conventionally, neodymium iron borate is generally recognized as one of the strongest, best performing hard magnet materials available. However, because this material is based on the rare earth element neodymium, it is expensive and often the available supply is not stable. Accordingly, there is a need for a material which performs equally or better than neodymium iron borate as a hard magnet but which is based on readily available and less expensive component materials.
Of various candidate materials under evaluation as a neodymium iron borate replacement, manganese bismuth alloy nanoparticles (MnBi) have been identified as a material of great interest.
Yang et al. (Applied Physics Letters, 99 082502 (2011)) and (Journal of Magnetism and Magnetic Materials, 330 (2013) 106-110) attributes many advantageous performance properties to low temperature phase manganese bismuth nanoparticles and describes the preparation of the MnBi nanoparticles by a melt spinning and annealing method. MnBi ingots were prepared by arc-melting and the ingot material was melted and the melt ejected onto the surface of a rotating copper wheel. After annealing, the obtained MnBi ribbons were ground to a powder having single crystal-like grains of a size of as little as 20-30 nm observed in the TEM image of the annealed. MnBi.
Suzuki et al. (Journal of Applied Physics 111, 07E303 (2012)) describe a study of the effect of mechanical grinding on the spin reorientation transition temperature (TSR) of MnBi prepared by melt spinning and annealing.
Iftime et al. (US 2012/0236092) describes core shell metal nanoparticles as a component of a phase change magnetic ink. MnBi is included as an example of a suitable core metal material. Preparation of such material is described in general as ball-milling attrition followed by annealing to effect crystallization of the amorphous milled product. No explicit description of the preparation of Mn Bi nanoparticles is provided and the Examples describe cobalt nanoparticle cores and iron nanoparticle cores.
Baker et al. (US 2010/0218858) describes permanent magnets of nanostructured Mn—Al and Mn—Al—C alloys. The nanoparticles are prepared by mechanical milling of the alloy metal and the resulting milled material is annealed. The initial alloy is prepared by melting a metal mixture and then quenching the melt,
Shoji et al. (US 2010/0215851) describes a method to produce core-shell composite nano-particles wherein the core particles are heated in advance of shell application. MnBi is listed as an example of a magnetic nanoparticle material. Although formation by a chemical synthesis method is indicated, no specific description of preparation of any alloy is provided.
Kitahata et al. (U.S. Pat. No. 6,143,096) describes a method to prepare a powder form Mn—Bi alloy wherein the raw materials are mixed and heated to a temperature higher than the melting points of the components; the powder obtained is thermally treated and then wet milled to obtain a powder having a particle diameter of less than 5 μm.
Kishimoto et al. (U.S. Pat. No. 5,648,160) describes a method for producing an MnBi powder wherein a Mn powder and a Bi powder are mixed. Both powders have a particle size of 50 to 300 mesh. The mixture is press molded and then thermally treated in a non-oxidizing or reducing atmosphere at a temperature not higher than the melting point of Bi. The Mn—Bi ingot is then ground to a particle size of from 0.1 to 20 μm.
Majetich et al. (U.S. Pat. No. 5,456,986) describes carbon coated Mn—Bi nanoparticles having a diameter of from 5 to 60 nm obtained by a carbon arc decomposition of graphite rods which are packed with manganese and bismuth.
None of these references describe or suggest a simple wet chemical method for the synthesis of MnBi nanoparticles having a particle size which is less than 20 nm. It is therefore an object of the present invention to provide a wet synthesis method to produce MnBi nanoparticles having a particle size of 20 nm or less.
It is a further object of the present invention to provide MnBi nanoparticles of the low temperature phase having a particle size of 20 nm or less.